1. Field
This document relates to an organic light emitting diode display, and more particularly, to an organic light emitting diode display, which can reduce image sticking caused by the deterioration of an organic light emitting diode, and a driving method thereof.
2. Related Art
Recently, organic light emitting diode displays spotlighted as display devices have the advantages of a rapid response speed, high emission efficiency, high luminance, and wide viewing angle by using a self-luminous device, which emits light by itself.
An organic light emitting diode display has an organic light emitting diode as shown in FIG. 1. The organic light emitting diode is provided with organic compound layers HIL, HTL, EML, ETL, and EIL formed between an anode and a cathode.
The organic compound layers comprise a hole injection layer HIL, a hole transport layer a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML to form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting diode display includes a plurality of pixels arranged in a matrix, each pixel including the organic light emitting diode. The organic light emitting diode controls the brightness of selected pixels in accordance with the gray scale of video data.
FIG. 2 equivalently shows one pixel in an organic light emitting diode display. Referring to FIG. 2, an pixel of an active matrix type organic light emitting diode display comprises an organic light emitting diode OLED, data lines DL and gate lines GL that cross each other, a switching thin film transistor SW, a drive thin film transistor DT, and a storage capacitor Cst. The switching TFT SW and the driving TFT DT may be a P-type MOSFET.
The switching TFT SW is turned on in response to a scan pulse received through the gate line GL, and thus a current path between a source electrode and a drain electrode of the switching TFT SW is turned on. During on-time of the switching TFT SW, a data voltage received from the data line DL is applied to a gate electrode of the driving TFT DT and the storage capacitor Cst. The driving TFT DT controls a current flowing in the organic light emitting diode OLED depending on a voltage difference Vgs between the gate electrode and a source electrode of the driving TFT DR. The storage capacitor Cst keeps a gate potential of the driving TFT DR during a frame period. The organic light emitting diode OLED may have a structure shown in FIG. 1. The organic light emitting diode OLED is connected between the source electrode of the driving TFT DT and a low potential driving voltage source VSS.
In general, non-uniformity between luminances of pixels occurs due to various causes, e.g., a difference in the electrical characteristics of driving TFTs, a difference in high potential driving voltage according to display positions, and a difference in the deterioration of organic light emitting diodes. Particularly, the difference in the deterioration of organic light emitting diodes occurs because the rate of deterioration varies from pixel to pixel in the case of long time driving. When this difference becomes severe, an image sticking phenomenon occurs. As a result, picture quality is deteriorated.
To compensate for the difference in the deterioration of the organic light emitting diodes, an external compensation technique and an internal compensation technique are known.
In the external compensation technique, a current source is placed outside a pixel, a constant current is applied to the organic light emitting diode via the current source, and then a voltage corresponding to the current is measured, thereby compensating for the difference in the deterioration of the organic light emitting diode. However, this technique requires all the parasitic capacitors of the data lines to be charged by current flowing in the data lines between the current source and the organic light emitting diode in order to sense an anode voltage of the organic light emitting diode, thus making the sensing speed very slow and lengthening the time required for the sensing. As a result, it is difficult to sense an anode voltage of the organic light emitting diode during time periods between adjacent frames or during the on/off of the display device.
In the internal compensation technique, a coupling capacitor is connected between the anode of the organic light emitting diode and a gate of the driving TFT to automatically reflect the degree of deterioration of the organic light emitting diode to a current flowing in the organic light emitting diode. However, with this technique, it is difficult to perform an accurate compensation because the magnitude of current is varied depending on the turn-on voltage of the organic light emitting diode using the current expression of the driving TFT, and a complicated pixel structure is required. Since the rate of deterioration of the organic light emitting diode is low, it is not necessary to compensate for the difference in the deterioration of the organic light emitting diodes while making the pixel structure complicated.